Depends. A pair of PB12-NSDs could potentially deliver smoother response over many seats thanks to modal averaging. I believe the dual NSDs would also deliver slightly more output than the single Plus, although that can vary in the real world. Conversely, the single plus can be tuned deeper if that's your cup of tea. Moving into the non-sonic items, would you rather move two ~60 pound boxes around or a single 125 pound box? Do you prefer two plain jane boxes or a single bigger but more attractive box?
I think you're definitely better off with two NSDs, provided that you locate and calibrate them well. To my mind this means locating them equidistant to the MLP because you don't have any hardware to set different distances for your two subs.
There's an extension advantage that the Plus affords over the NSD, but I suspect (and the crew at SVSound have confirmed) that this extension advantage is easily outdone by the frequency response advantage of having two or more subs. Me, I'm about to add a second NSD to my own room; I was considering either a factory return-upgrade or repurposing my NSD to a different room, but the SVSound folks made a credible case that doubling up on NSDs would be a better move.
Once again, I am sorry to take a sledgehammer to so small and fragile a nut. -- Richard Dawkins, The Greatest Show On Earth
So I guess my next question is, will Audessey Mult EQ XT handle the 2 subs properly with respect to EQ?
For what its worth, they will be placed the same listening distance from my main listening position. Also the will be symetrical across the front wall (if that makes any sense).
Xt? Not really. You will need a y adapter. But it will be fine. I could be wrong, but having the subs ~equidistant from lp if room is pretty much symmetrical by design and furniture might help best. But there are lots of other factors. If a receiver upgrade is in the cards look for one with Xt32, dual sub outputs and sub eq if possible.
My receiver is 7.2 currently so it has two sub outs. What is the question on the XT though. Not sure I understand the XT?, part.
Multi EQ XT32 with Sub EQ (Denon 4311, Onkyo 3009,5009) pings each subwoofer separately and sets delay, phase and level and then does a second sweep together after it sets them separate. XT does about the same but only calibrates the two subs as one it doesn't ping them separately. XT also doesn't have as many filters or resolution at XT32.